Phase locked loops are used primarily for locking on and holding a frequency of an oscillator with respect to a fixedly determined reference frequency. A high frequency stability and phase stability of the oscillator are achieved thereby. One example of a phase locked loop is included in: Gehrung, Bressler, Gutekunst, “Elektronik für Ingenieure” [Electronics for Engineers], 3rd edition, Springer, 1998, on pages 509-519, which schematically shows a basic circuit of a phase locked loop, called PLL for short.
The PLL shown therein comprises a forward path having a phase comparator, a low-pass filter and also the oscillator to be regulated, and a backward path having a frequency divider. An important constituent part is the phase comparator, which compares the fed-back signal UN with a reference signal URef and generates a control signal UUPD therefrom. In the exemplary embodiment, said control signal is forwarded to a low-pass filter, at the output of which a control voltage UTP readjusts the frequency of the voltage-controlled oscillator embodied in the present example. The output signal of the oscillator VCO is divided by means of the frequency divider N in the backward path and supplied again to the feedback input of the phase comparator.
In this case, the regulating signal UPD of the phase comparator PD may be present for example in the form of signal pulses of different length with a positive or negative amplitude, depending on the phase shift between the reference signal URef and the fed-back and divided signal UN of the oscillator VCO.
If the frequency of the fed-back signal UN is equal to the frequency of the reference signal URef and if the signals have a fixed phase relationship with respect to one another, then the phase detector no longer generates a control signal UUPD and the voltage-controlled oscillator is locked on. However, small current and voltage fluctuations in the oscillator generate a phase noise in the output signal, thereby widening the spectrum of the oscillator signal. This widening is partly corrected by the phase locked loop. In this case, the regulating bandwidth results from the charge pump operating as a low-pass filter.
The measurement of the phase noise of a phase locked loop therefore permits conclusions to be drawn regarding a quality factor and the performance of the phase locked loop. However, a phase noise measurement of a phase locked loop is possible only with great complexity.